The present invention generally relates to surface mount (SM) circuit devices that are attached to conductor patterns with solder connections formed by reflow soldering. More particularly, this invention relates to a method for controlling the height of such solder connections, and preventing shorting between adjacent connections.
A flip chip is generally a monolithic surface mount (SM) semiconductor device, such as an integrated circuit, having bead-like terminals formed on one of its surfaces. The terminals, typically in the form of solder bumps, serve to both secure the chip to a circuit board and electrically interconnect the flip chip circuitry to a conductor pattern formed on the circuit board, which may be a ceramic substrate, printed wiring board, flexible circuit, or a silicon substrate. Due to the numerous functions typically performed by the microcircuitry of a flip chip, a relatively large number of solder bumps is required. The solder bumps are typically located at the perimeter of the flip chip on electrically conductive pads that are electrically interconnected with the circuitry on the flip chip. The size of a typical flip chip is generally on the order of a few millimeters per side, resulting in the solder bumps being crowded along the perimeter of the flip chip.
Because of the narrow spacing between adjacent solder bumps and conductors, soldering a flip chip to its conductor pattern requires a significant degree of precision. Reflow solder techniques are widely employed for this purpose, and typically entail precisely depositing a controlled quantity of solder on the flip chip using methods such as electrodeposition. Once deposited, heating the solder above its liquidus temperature serves to form the characteristic solder bumps on the surface of the flip chip. After cooling to solidify the solder bumps, the chip is soldered to the conductor pattern by registering the solder bumps with their respective conductors and then reheating, or reflowing, the solder so as to metallurgically adhere, and thereby electrically interconnect, each solder bump with its corresponding conductor, forming what will be referred to herein as a solder connection.
Placement of the chip and reflow of the solder must be precisely controlled not only to coincide with the spacing of the terminals and the size of the conductors, but also to control the height of the solder connections after soldering. As is well known in the art, controlling the height of solder connections after reflow is often necessary to prevent the surface tension of the molten solder from drawing the flip chip excessively close to the substrate during the reflow operation. Sufficient spacing between the chip and its substrate, often termed the stand-off height, is desirable for allowing penetration of cleaning solutions for removing undesirable processing residues, promoting the penetration of mechanical bonding and encapsulation materials between the chip and its substrate, and enabling stress relief during thermal cycles. Solder bump position and height are generally controlled by the amount of solder deposited on the flip chip to form the solder bump and/or by the use of solder stops that limit the surface area over which the solder bump is allowed to reflow. Solder stops are typically formed by a solder mask on laminate substrates and printed dielectric on ceramic substrates. Because flip chip solder bumps are registered and soldered directly to their conductors, the conductors must be formed of a solderable material, which as used herein means that a tin, lead or indium-based alloy is able to adhere to the conductor through the formation of a metallurgical bond. Solder stops are intentionally formed of a nonsolderable material, meaning that a tin, lead or indium-based solder will not adhere to the material for failure to form a metallurgical bond. Upon reflow, the reflow area defined by a solder stop on a conductor yields a solder connection typically having a semi-spherical shape and circular cross-section.
While solder stops are widely used in the art, trends in the industry have complicated their ability to yield solder connections that provide an adequate flip chip stand-off height. As flip chips have become more complex, the number of bumps that must be accommodated along the chip perimeter has increased. In turn, the conductors to which the bumps are registered and soldered have become more closely spaced and narrower, e.g., a pitch of about 0.010 inch (about 0.25 millimeter) or less and line widths of about 0.004 inch (about 0.1 millimeter), yielding a line spacing of about 0.006 inch (about 0.15 millimeter) or less. Fine conductor pitches complicate the design and fabrication of solder stops, particularly on laminate substrates with the result that pitches of less than 0.010 inch have not been widely used. As a result, solder connections having adequate stand-off height are more difficult to consistently produce, which increases the difficulty of adequately dispersing encapsulation materials between flip chips and their substrates. Shorts between adjacent solder connections are also more likely to occur due to excessive lateral flow of the solder during reflow.
Accordingly, it would be desirable if an improved method were available that could control the stand-off height of a surface mount device following solder bump reflow, while also reducing the incidence of shorting between solder connections if the device must be registered with a fine pitch conductor pattern.
The present invention provides a method for controlling the shape and height of solder connections of a surface mount circuit device, such as a flip chip, by way of controlling the manner and extent to which solder is able to flow on a conductor during reflow. Solder connections formed by the method of this invention are characterized by having shapes that promote maximize the distance between adjacent connections so that the incidence of shorting between solder connections is reduced, while achieving a suitable stand-off height for the device.
According to this invention, the above is achieved by forming on a circuit board a conductor pattern defined by a number of conductors, each having a reduced-width portion. A mask is formed on the circuit board to have an opening that exposes at least a portion of each of the reduced-width portions of the conductors, with each exposed portion having opposing first and second ends. A circuit device having a staggered pattern of solder bumps is then placed on the circuit board so that each solder bump is registered with one of the exposed portions of the conductors. More particularly, the solder bumps are staggered on the circuit device so that every other solder bump is registered with a first end of one of the exposed portions, and so that the remaining intervening solder bumps are registered with the opposing second ends of the remaining exposed portions. The solder bumps are then reflowed to form solder connections between the circuit device and the conductors. During reflow, the solder bumps registered with the first ends of the exposed portions flow toward the corresponding second ends of the exposed portions, and the solder bumps registered with the second ends of the exposed portions flow toward the corresponding first ends of the exposed portions.
The result of the method described above is that the solder connections are wider at the end of the exposed portion with which its corresponding solder bump was registered, and the width of each solder connection is tapered to become narrower toward the opposing end of its exposed portion. Because of the original staggered arrangement of the solder bumps, the solder connections of every other conductor are tapered in a first direction, and the remaining solder connections of the intervening conductors are tapered in the opposite direction. For a given conductor pitch, the invention achieves a relatively greater distance between solder connections than possible with conventional solder connections having circular cross-sections, with the result that the likelihood of shorting between adjacent solder connections is significantly reduced. In addition, adequate stand-off height is maintained by the wider ends of the solder connections to allow penetration of cleaning solutions, promote the penetration of mechanical bonding and underfill materials between the device and its substrate, and promote stress relief in the solder connections during thermal cycling.
Other objects and advantages of this invention will be better appreciated from the following detailed description.